1. Field of the Invention
The present invention relates to a semiconductor stack including a semiconductor element such as a thyristor and a gate turnoff element and a fuse connected to the semiconductor element in series for protecting the semiconductor element from a breakdown due to an overcurrent.
2. Description of the Prior Art
In a conventional semiconductor converter, there have been a variety of countermeasures for protecting a semiconductor element such as a thyristor and a gate turnoff element from a breakdown caused by an overcurrent. For instance, a fuse has been widely used for preventing the overcurrent of the semiconductor element because of an simple and exact means as compared with other methods.
Recently, a capacity of a semiconductor element has been remarkably enlarged, for example, a semiconductor element having a rated current of several thousands of amperes has been used. Accordingly, a protective fuse having a large capacity has been demanded. However, there is no fuse having the same capacity as that of such a semiconductor element having an enlarged capacity, and hence, in practice, a plurality of fuses connected in parallel are used.
Conventionally, when a plurality of fuses are connected in parallel to one another, as shown in FIG. 1, a pair of conductor couplers 2 having a L-shaped cross section are attached to both ends of each of fuse members 1, and then the couplers 2 attached to the fuse members 1 are secured to a pair of conductor plates 3a and 3b. Alternatively, as shown in FIG. 2, both the ends of the fuse members 1 are directly secured to the conductor plates 3a and 3b without using the couplers 2.
In this case, generally, the connected fuse members 1 are so mounted in proximity to a semiconductor stack as to be readily exchanged with other ones while one of the conductor plates 3a and 3b is connected to the semiconductor stack.
In the first conventional method for connecting the fuse members 1 to the conductor plates 3a and 3b using the couplers 2, as shown in FIG. 1, approximately a double space for connecting is required as compared with the second conventional method for connecting the fuses 1 the conductor plates 3a and 3b without using the couplers 2, as shown in FIG. 2. Particularly, when the fuses 1 are directly connected to the conductor plates 3a and 3b in parallel without using the couplers 2, as shown in FIG. 2, in fact, a tensile stress is often given to the fuses 1 due to the differences among the lengths of the fuses 1 or the differences among the parallelism of the contact end surfaces of the fuses 1. As to the fuse 1 now practically used, the external periphery is made of a ceramic material or the like, and thus the fuse 1 is very fragile against the tensile stress. Therefore, the fuses 1 have been sometimes broken down by the tensile stress.
Further, in the conventional converter, when the semiconductor stack and the fuses are separately mounted, a particular conductor member is required to connect these members. In addition, since it is necessary that the fuses are so mounted as to readily replaced by other ones, the space for mounting the fuses becomes necessarily large, and hence the apparatus cannot be miniaturized.
Further, since a heat loss generated from the fuses is large, it is often necessary to cool the surfaces of the fuses. When the surfaces of the fuses are cooled, their insulator portions made of the ceramic material or the like are cooled by using an airflow having a several m/sec flow speed, or the temperature of the contact surfaces of the fuses is reduced to below a certain value by using a heat sink.
In general, a cooling type of the fuses is determined depending on a cooling type of the semiconductor element, that is, when an air-cooling semiconductor element is used, the insulator portions of the fuses are cooled by an airflow, or, when a water-cooling semiconductor element is used, the contact surfaces of the fuses are cooled by using a water-cooling heat sink. When the insulator portions of the fuses are air-cooled, a small cooling fan is arranged in front of the fuses or a cooling fan installed for cooling the semiconductor element and the like is utilized.
In the conventional semiconductor converter having the air-cooling fuses and the small cooling fan arranged in front of the fuses, the small cooling fan is necessarily arranged in close proximity to the fuses in order to obtain the necessary airflow speed such as several m/sec. Accordingly, in a conventional semiconductor converter in which a plurality of fuses are dispersedly arranged, at least the same number of the cooling fans as the dispersed number of the fuses are required. Since the cooling fan includes a rotating machine, abrasion and vibration are generated, and a failure rate of the cooling fan is high as compared with the parts of the semiconductor element. Further, the life of the cooling fan is short such as several years. Therefore, the increase of the number of the cooling fans brings a drop of the reliability of the whole apparatus. Furthermore, since a space for mounting the cooling fans is required, the entire apparatus is enlarged and accordingly the apparatus cannot be miniaturized.
In the conventional semiconductor converter including the air-cooling fuses and the cooling fan installed for cooling the semiconductor element, since the semiconductor stack and the fuses are arranged in the separate positions, the structure of the apparatus becomes complicated or the exchange of the fuses cannot be conducted so easily in order to flow the cooling air towards the fuses.